1. Field of the Invention
The present invention relates to a signal receiving apparatus, a signal receiving method adopted by the apparatus, a signal receiving program implementing the method and a signal receiving system employing the apparatus. To put it more specifically, the present invention relates to a signal receiving apparatus capable of efficiently demodulating a signal transmitted by adoption of a MISO (Multi Input Single Output) method and relates to a signal receiving method adopted by the apparatus, a signal receiving program implementing the method as well as a signal receiving system employing the apparatus.
2. Description of the Related Art
In recent years, there has been used a modulation method referred to as an OFDM (Orthogonal Frequency Division Multiplexing) method as a method for transmitting a digital signal. The OFDM method is a method for carrying out digital modulation based on a PSK (Phase Shift Keying) technique or a QAM (Quadrature Amplitude Modulation) technique by which a number of orthogonal subcarriers are prepared in the transmission band and data is allocated to the amplitude and phase of each of the subcarriers. An OFDM time area signal is transmitted in symbol units each referred to as an OFDM symbol.
There are a number of cases in which the OFDM method is applied to terrestrial wave digital broadcasting much affected by multipath obstacles. There are specifications for the terrestrial wave digital broadcasting based on the OFDM method. Typical examples of such specifications are the DVB-T (Digital Video Broadcasting-Terrestrial) and the ISDB-T (Integrated Services Digital Broadcasting-Terrestrial).
By referring to a block diagram of FIG. 1, the following description explains a typical configuration of a signal receiving apparatus 1 for receiving a signal modulated by adoption of the OFDM method.
The typical configuration of the signal receiving apparatus 1 shown in the block diagram of FIG. 1 includes an antenna 11, an AD (Analog-to-Digital) conversion section 12, an FFT (Fast Fourier Transform) section 13, an equalization section 14 and an error correction section 15.
The antenna 11 receives a digital broadcast signal transmitted by a signal transmitting apparatus 2 for example installed in a broadcast station and supplies the signal to the FFT section 13 by way of the AD conversion section 12. In accordance with a trigger-position command received from the equalization section 14, the FFT section 13 carries out an FFT calculation and supplies a signal obtained as a result of the FFT calculation to the equalization section 14.
The equalization section 14 extracts an SP (Scattered Pilot) signal from the signal obtained as a result of the FFT calculation and finds the profile of a transmission line between the signal receiving apparatus 1 and the signal transmitting apparatus 2 as well as other information by making use of the SP signal in order to carry out signal demodulation processing. The profile of a transmission line is the characteristic of the transmission line. To be more specific, the profile of a transmission line shows a response to an impulse input in a time region of the transmission line.
The SP signal is a scattered pilot signal used by the signal receiving apparatus 1 for inferring the transmission-line characteristic which is the frequency characteristic of the transmission line. As symbols forming an OFDM transmission frame, there are SP signals in addition to data carriers for conveying data. That is to say, each SP signal is also assigned to a carrier.
FIG. 2 is a diagram showing a typical signal-location pattern of SP signals among OFDM symbols. Each of FIGS. 3 and 4 is a diagram showing a typical signal-location pattern obtained as a result of a time interpolation process carried out by making use of SP signals existing among the OFDM symbols shown in the diagram of FIG. 2. FIG. 5 is a diagram showing a typical signal-location pattern obtained as a result of a spatial interpolation process carried out on SP signals existing among the OFDM symbols shown in the diagram of FIG. 2. It is to be noted that, in each of the diagrams of FIGS. 2 to 5, the horizontal axis represents carriers of OFDM signals whereas the vertical axis represents OFDM symbols of the OFDM signals. A carrier number is assigned to every carrier whereas a symbol number is assigned to each symbol even though neither the carrier numbers nor the symbol numbers are shown in the diagrams. The carriers correspond to the frequency whereas the symbols correspond to the time.
In each of the signal-location patterns shown in the diagrams of FIGS. 2 to 5, every circle represents an OFDM symbol. A white circle indicates (a carrier of) data serving as the subject of transmission. In some cases, the data includes a TMCC (Transmission and Multiplexing Configuration Control) pilot signal and an AC pilot signal. On the other hand, a black circle indicates an SP signal. In each of the signal-location patterns shown in the diagrams of FIGS. 3 to 5, each densely hatched circle indicates data obtained as a result of a time interpolation process carried out by making use of SP signals. In the following description, the data obtained as a result of a time interpolation process carried out by making use of SP signals is referred to as a time-interpolation SP. In the diagram of FIG. 5, every dashed-line circle indicates data obtained as a result of a frequency interpolation process carried out by making use of time-interpolation SPs which include SP signals. In the following description, the data obtained as a result of a frequency interpolation process carried out by making use of time-interpolation SPs is referred to as a frequency-interpolation SP.
The SP signal is a complex vector which has a known amplitude and a known phase. For example, in an OFDM transmission frame, an SP signal is provided for every 3 carriers. A data carrier for conveying data serving as the subject of the transmission is provided between SP signals. The signal receiving apparatus 1 receives an SP signal in a state of being distorted due to the effect of the characteristic of the transmission line. The SP signal prevailing at the signal receiving time is compared with an SP signal known at the signal transmitting time in order to obtain the transmission-line characteristic at the position of the SP signal.
On the basis of characteristics exhibited by the transmission line at the positions of the SP signals, the equalization section 14 carries out an interpolation process in the time direction of carriers, among which SP signals are located, for every symbol. The characteristic exhibited by the transmission line at the position of an SP signal has been obtained as a result of comparing the SP signal with an SP signal known at the signal transmitting time. As a result of the interpolation process carried out in the time direction, the equalization section 14 generates time-interpolation SPs. Each of the generated time-interpolation SPs is indicated by a densely hatched circle shown in the diagram of FIG. 3. The equalization section 14 then compares the data shown in the diagram of FIG. 2 as data at the signal receiving time with the time-interpolation SPs shown in the diagram of FIG. 3 in order to infer the characteristic of the transmission line for each symbol. As a result, for all symbols, the characteristic of the transmission line is, inferred for every 3 carriers laid out in the frequency direction, and a transmission-line profile showing the inferred characteristic of the transmission line is derived from the characteristic of the transmission line. The profile of a transmission line is used in processing such as a process of finding a trigger position of an FFT calculation.
Then, the equalization section 14 makes use of the time-interpolation SPs including SP signals in order to carry out an interpolation process in the frequency direction as shown in the diagram of FIG. 4. That is to say, the equalization section 14 implements a frequency-interpolation filter at every candidate center position for the time-interpolation SPs including SP signals in order to generate frequency-interpolation SPs. Then, the equalization section 14 compares the frequency-interpolation SPs with signals known at the signal transmitting time in order to find an optimum center position of a frequency-interpolation filter. A typical example of the known signals compared with the frequency-interpolation SPs is TMCC pilot signals.
Then, at the optimum center position, the equalization section 14 implements a frequency-interpolation filter in order to carry out an interpolation process in the frequency direction for the time-interpolation SPs including SP signals. As a result of the interpolation process carried out in the frequency direction, the equalization section 14 generates frequency-interpolation SPs as shown in the diagram of FIG. 5. In this way, a channel characteristic is inferred. The channel characteristic is the characteristic of the transmission line in the frequency direction of all carriers. Subsequently, the equalization section 14 divides a signal obtained as a result of an FFT calculation carried out by the FFT section by the channel characteristic in order to carry out an equalization process on a signal received from the signal transmitting apparatus 2. Finally, the equalization section 14 supplies a signal obtained as a result of the equalization process to the error correction section 15.
The reader is suggested to refer back to the block diagram of FIG. 1. The error correction section 15 carries out de-interleave processing on a signal interleaved by the signal transmitting apparatus 2 in order to generate a TS (Transport Stream) of decoded data which is obtained as a result of processes which include a de-puncture process, a Viterbi decoding process, a scattered-signal elimination process and an RS (Reed Solomon) decoding process. Then, the error correction section 15 supplies the resulting decoded data to for example an output section provided at a later stage shown in none of the figures.
By the way, in May 2009, the ETSI (European Telecommunication Standard Institute) was formulating DVB (Digital Video Broadcasting)-T.2 as the standards of the terrestrial digital broadcasting of the next generation. For more information on these standards, the reader is suggested to refer to DVB BlueBook A122 Rev. 1, Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T.2), Sep. 1, 2008, DVB home page, search on May 18, 2009 at an Internet address <URL: http://www.dvb.org/technology/standards/>.